Per-grain and neighbourhood stress interactions during deformation of a ferritic steel obtained using three-dimensional X-ray diffraction

Abstract: Three-dimensional X-ray diffraction (3DXRD) has been used to measure, in-situ, the evolution of $\sim 1800$ grains in a single phase low carbon ferritic steel sample during uniaxial deformation. The distribution of initial residual grain stresses in the material was observed to prevail as plasticity builds, though became less pronounced, and therefore less influential as strain increased. The initial Schmid factor of a grain was found to be strongly correlated to the intergranular stress change and the range of stresses that are permissible; a grain well aligned for easy slip is more likely to exhibit a range of stresses than those orientated poorly for dislocation motion. The orientation path of a grain, however, is not only dependent on its initial orientation, but hypothesised to be influenced by its stress state and the stress state of its grain environment. A grain neighbourhood effect is observed: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are much less influential. This phenomenon is strongest at low plastic strains only, with the effect diminishing as plasticity builds. The influence of initial residual stresses becomes less evident, and grains rotate to eliminate any orientation dependent load shedding. The ability of the BCC ferrite to exhaust such neighbourhood interactions, which would otherwise be detrimental in crystal structures with lower symmetric and fewer slip systems, is considered key to the high ductility possessed by these materials.